The present invention relates to a superluminescent diode which is effectively used as a light source for an optical fiber gyro, optical disk, or the like, and can radiate coherent light with high intensity and small radiation angle.
In a superluminescent diode for deriving a large coherent light output from an end face of an active layer, it is important to suppress laser oscillation in a Fabry-Perot (FP) mode. In order to suppress this oscillation, a countermeasure for reducing a light emission factor at the end face of the active layer such as end face AR (anti-reflecting) coat, formation of a non-excitation region, oblique etching of an end face, end face burying, or the like has been taken. However, it is difficult to sufficiently suppress FP mode oscillation using only the AR coat. When FP mode oscillation is suppressed by obliquely etching the end face or burying the end face, or a combined means, a refractive index at the end face becomes unexpectedly large, and a reflectivity reaches about 1% as compared to the case of cleavage. In particular, when the thickness of the active layer is increased, this influence becomes noticeable, and the reflectivity is also increased. Thus, FP mode oscillation is difficult to suppress using only the above-mentioned means.
FIGS. 1a to 1c show a buried superluminescent diode in which a non-excitation region 10 combined with an end face burying region 11 is formed. Reference numeral 9 denotes a current injection region; 10, a non-excitation region; and 11, an end face burying region. Reference numeral 13 denotes an AR coat formed on a light output surface. Since the width of an active layer 3 of the non-excitation region 10 is the same as that of the current injection region 9, light is effectively guided. Therefore, carriers are excited in the non-excitation region 10, and an absorption coefficient is reduced thereby. Therefore, in order to suppress FP mode oscillation, the length of the non-excitation region 10 must be several times longer substantially than that of the current injection region 9, i.e., must be elongated.
FIG. 2 is a plan view of a buried superluminescent diode in which a non-excitation region which can solve the drawback of the diode shown in FIGS. 1a to 1c to some extent is formed. The feature of this diode is that the axial orientation of a cleavage end face and those of the active layers of the current injection region 9 and the non-excitation region 10 are offset from a vertical direction, and both the end faces do not easily serve as a resonator in laser oscillation. A suppression effect of FP mode oscillation is enhanced as the offset in the axial orientation of the active layer is larger at both the end faces and the reflected light at the end faces goes outside the waveguide. However, since a light output direction is not perpendicular to the end face, it is difficult to obtain a high coupling efficiency in consideration of coupling with an optical fiber.
In the conventional superluminescent diode with the above-mentioned structure, when a light guide layer is formed adjacent to the active layer in order to improve a light extraction efficiency, as shown in FIGS. 1a to 1c, an effective absorption coefficient of the non-excitation injection region is decreased, and FP mode laser oscillation tends to occur.